This does not implement all opcodes related to div/sqrt as specified in
the xtensa ISA, partly because the official specification is not
complete and partly because precise implementation is unnecessarily
complex. Instead instructions specific to the div/sqrt sequences are
implemented differently, most of them as nops, but the results of
div/sqrt sequences is preserved.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>